Polyolefin-based resins generally show moderate heat resistance and high solvent resistance. This feature makes the compounds suitable for use in a wide range of applications, including wire coatings; cable coatings; tubes; films, such as laminate films, agricultural films, stretch wrapping films, adhesive films, and medical films; sheets, such as agricultural sheets; processed paper; injection molding; pipes; bundling tapes; flat yarns; and fibers. Nonetheless, as far as such applications as wire coatings, tubes, films, and sheets are concerned, further improvements are required in certain physical properties of polyolefin-based resins, including heat resistance and solvent resistance.
The following several methods or techniques have been proposed in an effort to impart required mechanical properties, heat resistance and solvent resistance to the polyolefin-based resins. In a method <1>, a crosslinkable polymer composition composed of a polyolefin-based resin and a crosslinkable monomer is exposed to an active energy ray (Japanese Patent Laid-Open Publication No. Hei 10-147671). In another method <2>, polyethylene is blended with an ethylene-α-olefin copolymer or a styrene-based block copolymer (Japanese Patent Laid-Open Publication No. Hei 11-130921).
Also, in a technique <3>, a polyethylene resin composition that undergoes crosslinking upon exposure to electron beam has been proposed. This composition is composed of a polyethylene and a highly crosslinkable ethylene copolymer obtained by copolymerization of a non-conjugated diene having 8 or more carbon atoms (Japanese Patent Laid-Open Publication No. Hei 7-48482). In a technique <4>, a molded article has also been proposed that comprises an ethylene interpolymer (preferably, uniformly branched ethylene polymer) copolymerized with at least one other monomers and that is formed by curing, irradiating, or crosslinking the ethylene polymer (National Publication of PCT application No. 2002-515530). In a method <5>, also proposed is a crosslinked composition containing silane-modified polyolefins and polypropylenes (International Patent Publication No. WO 01/038433).
Furthermore, in a technique <6>, a method for producing a particular elastomer has been proposed. In this method, a composition containing an ethylene-propylene rubber, a crystalline polymer (preferably, polyethylene) and an inorganic filler is exposed to an ionizing radiation to cause the composition to undergo crosslinking and thus form the elastomer (Japanese Patent Laid-Open Publication No. Sho 51-59981). In a technique <7>, a composition has also been proposed that has been improved in its properties such as oil-resistance, cut-through property, thermal deformation, and solder resistance. This composition is obtained by crosslinking a particular crosslinkable resin composition either chemically or by exposure to an electron beam. The crosslinkable resin composition is composed of a styrene-ethylene/butylene-styrene block copolymer, a polypropylene, an oil, a stabilizer, and a crosslinking aid (Japanese Patent Laid-Open Publication No. 58-145751). In a technique <8>, a heat-resistant and flame-resistant crosslinkable polyethylene composition has been proposed that is obtained by causing a particular composition to undergo crosslinking either chemically or by exposure to an electron beam. The composition is composed of a polyethylene, a styrene-ethylene/butylene-styrene block copolymer, and a flame retardant (Japanese Patent Laid-Open Publication No. Sho 59-105040).
However, the method <1>, which involves addition of a crosslinkable monomer and irradiation with an active energy ray, cannot necessarily provide the resulting crosslinkable polymer composition with sufficient heat resistance or solvent resistance. In the method <2>, in which a polyethylene is mixed with an ethylene-α-olefin copolymer or a styrene-based block copolymer, the addition of ethylene-α-olefin copolymer does not significantly improve the heat resistance of the resulting resin, while the addition of styrene-based block copolymer often results in a reduced solvent resistance of the resin. Thus, the method is not appropriate when it is desired to obtain a composition that exhibits flexibility and heat resistance in a well-balanced manner.
Although each of the techniques <3> through <5> is effective in facilitating crosslinking of the resin, as well as in improving the heat resistance of the resin, none is effective in improving the flexibility of the resin and is thus suitable for applications that require high flexibility.
Involving an ethylene-propylene copolymer rubber or a styrene-ethylene/butylene-styrene block copolymer as an elastic component, the techniques <6> through (8) are each effective to some extent in increasing the flexibility of the resin. However, each of these techniques fails to impart sufficient crosslinkability and, thus, heat resistance to the resin and is not suitable for use in applications that require high heat resistance, such as wire coatings intended for use in the engine room of automobiles.
For these reasons, a demand exists for a polyolefin-based resin composition that exhibits flexibility, mechanical properties, heat resistance, and solvent resistance in a well-balanced manner and is thus suitable for use in applications such as wire coatings, cable coatings, tubes, films, and sheets.